This invention relates to a process for making an article suitable for use as a composite catalyst body. More particularly, it relates to a process whereby an aluminum substrate is anodized, a metal is electrolytically deposited at the base of the pores of the oxide, and the oxide is then partially stripped such that the deposited metal is exposed at the surface of the remaining oxide layer. The electrolytically deposited metal may be a catalytic metal or may be comprised of a base metal onto which a catalytic metal is deposited.
The coloring of aluminum by anodizing to form a porous oxide and subsequently electrolytically depositing a metal within the pores is well known. A basic discussion can be found in xe2x80x9cThe Surface Treatment and Finishing of Aluminum and its Alloysxe2x80x9d authored by S. Wernick, P. Pinner and P. G. Sheasby, fifth edition, copyrighted 1987 by Finishing Publications Ltd. and co-published in the United States with ASM International, Metals Park, Ohio, on pages 600 to 625 of volume 1. In such processes, a porous oxide film is first produced using direct current or alternating current in an anodizing electrolyte of the type known to produce such oxides. Examples of electrolytes would include sulfuric acid and phosphoric acid. The anodized carrier is subsequently treated electrolytically to deposit a metal within the pores of the oxide.
The deposited metals would include, for example, nickel and copper.
This is most often accomplished using alternating current and the electrolyte solution contains a salt of the metal to be deposited. The counter electrode may be either an inert material such as graphite or alternatively it may be, for example, a nickel or copper electrode. The metal is known to be deposited at the base of the pores in the oxide film.
U.S. Pat. Nos. 3,929,594 and 4,021,592 disclose an electroplated anodized aluminum article and a process for making such an article wherein an aluminum substrate has an unsealed porous layer formed on the substrate and then has a metal electrolytically deposited thereon to form discete metal islands having a root portion anchored in the pores of the oxide layer. The islands extend from the root portion above the surface of the oxide layer in a bulbous, undercut configuration. It is disclosed that the article in these patents can be a composite catalyst body by using a catalytically active metal to form the islands.
The previously mentioned co-pending patent application Ser. No. 08/502,121 discloses the formation of a catalytic surface wherein a base metal such as aluminum is anodized to form a porous unsealed oxide coating. Nodules are then formed on the surface with roots anchored in the pores of the coating by electrodepositing a relatively inexpensive core metal such that the nodules extend above the surface in a bulbous, undercut configuration. A second catalytic metal is subsequently electrodeposited onto the surface of the core metal nodules. The catalytically coated nodules are optionally removed from the oxide to form discrete catalyst coated particles.
The present invention is directed to the formations of a catalytic surface wherein an aluminum substrate is anodized to form a porous, unsealed oxide coating. A metal is then electrolytically deposited within the pores of the oxide, and the oxide is subsequently partially stripped to expose the metal deposit at the surface. In the case where the metal is a catalytically active metal, the article thus formed may serve as a composite catalyst body. Alternatively, the electroltic deposition process may be carried out in two stages where a base metal is first deposited within and closest to the base of the pores. A second, catalytically active metal is subsequently deposited within the pores on top of the base metal at the base of the pores. The oxide can then be sufficiently stripped to expose the catalytically active metal at the surface.
In an alternative embodiment, the metal deposited within the pores is a base metal. The oxide is then sufficiently stripped to expose the base metal at the surface. A catalytically active metal is then deposited onto the exposed base metal at the surface. This deposition of the catalytically active metal onto the base metal may be accomplished by electrolytic deposition or by electroless chemical plating. The deposition is typically carried out such that the catalytic metal sites remain separate and discrete. Alternatively, the deposition may be allowed to proceed such that the catalytic metal bridges the gaps between the base metal to cover the entire-surface. The resultant article would thus have a catalyst layer over the entire surface anchored by the base metal roots in the pores.
The process of the invention may be further carried out with the inclusion of an additional anodizing process. After the metal is deposited into the pores, the substrate is then subjected to an additional anodizing process. Since the anodizing process produces the oxide at the interface between the aluminum and the oxide layer, this further oxidation occurs beneath the deposited metal. The article is then subjected to a partial stripping process such that the deposited metal is exposed at the surface.
Also, this second anodizing process may be carried out in an electrolyte such that the dissolution of the oxide layer in which the metal is deposited is caused to occur sufficiently such that the metal is exposed at the surface at the same time as additional oxide is being formed.
Further, the process of the present invention may optionally include the step of increasing the cross section of the pores at the region of the base prior to the electrolytic deposition of the metal.
Finally, the process of the invention may be carried out so as to form discrete catalytic particles.